Could how high people live affect their weight?

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Behind the Headlines

Thursday February 14 2013

Can living in the mountains make you slimmer?

Could how high people live affect their weight? Do slimmers need to head for the hills? Reuters has reported on a new study that suggests that people who dwell at high altitude are less likely to be obese.

The research found that people who lived at less than 500m above sea level (such as New Yorkers) were much more likely to be obese than people who lived 3,000m or more above sea level (such as people who lived in Denver, Colorado).

Even after taking into account factors that may be associated with life at higher altitudes, such as increased physical activity (possibly due to more climbing) and colder temperatures, there was still a significant link between higher altitudes and obesity rates.

Researchers found that men living at altitudes below 500m were 5.1 times more likely to be obese compared with their counterparts living above 3,000m. Meanwhile, women living at these low levels were 3.9 times more likely to be obese.

While the researchers can’t pin down the exact cause of this relationship, they speculate that low oxygen levels at high altitude, which increase energy demands and potentially influence foetal and child development, may be responsible. However, it is likely that the connection between altitude and obesity is part of a complex relationship between biology, demographics, environment and lifestyle factors.

Where did the story come from?

How do other countries match up?

If there were a direct link between obesity and altitude it could be assumed that mountainous countries would have low levels of obesity. However, comparing mountainous countries with low-lying ones gives inconclusive answers.

The notoriously flat Netherlands has an obesity rate of 11.8%, according to the Organisation for Economic Co-operation and Development (OECD). This is at the low end of the scale. Meanwhile, neighbouring Luxembourg, which is marginally more hilly, but still relatively low, has a much higher obesity rate of 22.1%, according to the OECD.

Mexico City, on the other hand, with an average altitude of more than 2,000 metres, has a 'massive obesity rate' with around two-thirds of adults being obese.

This would suggest that the same link between altitude and obesity may not be seen in other countries, and further study outside the US would provide a useful comparison.

The study was carried out by researchers from the University of the Health Sciences, Bethesda, and Virginia Commonwealth University and Obetech Obesity Research Center, Richmond, USA. No sources of financial support are reported.

The study was published in the peer-reviewed medical journal International Journal of Obesity.

What kind of research was this?

The researchers highlight the observed trend that, in the US, obesity seems to be most prevalent in the southeastern states and Midwest, and less so in the ‘mountain west’ states. They say that differences in elevation provide a biologically plausible explanation, with suggested theories including increased metabolic demand and reduced childhood growth in response to altitude.

However, other observational studies of different populations worldwide have given varied results. For example, people in Peru have higher than average rates of obesity-related diseases despite living at a higher altitude.

This cross-sectional study aimed to look at the geographic distribution of obesity across the US and see how it related to elevation level, temperature and urbanisation, while also adjusting for other behavioural and demographic factors.

Such a study can demonstrate an association between obesity levels and altitude. But it cannot prove that altitude has a direct effect on BMI or say what biological process causes this.

What did the research involve?

This research used 2011 data gathered from the Behavioral Risk Factor Surveillance System (BRFSS), which is said to be a nationwide telephone health survey representative of the US population.

The collected data included information on diet and physical activity and demographic details (age, sex, race or ethnicity, education and income). Obesity was defined as body mass index (BMI) of 30 kg/m2 or greater – which is an internationally agreed definition.

Elevation above sea level, average annual temperature and urbanisation for participants was based on their county of residence reported in the 2011 survey. They had these data for 3,134 administrative areas (counties) within the US.

The researchers used statistical methods to look at the association between obesity and elevation above sea level, average annual temperature and urbanisation, taking into account the demographic and lifestyle factors data they had.

What were the basic results?

The researchers had full data available for 422,603 US citizens. The researchers found that, compared with the 322,681 people at the lowest level of elevation (less than 500m above sea level) those 236 people at the highest level of elevation (3,000m or more above sea level) were less likely to smoke and were more likely to comply with physical activity and diet recommendations.

After taking into account temperature, urbanisation, demographic factors and lifestyle factors (such as physical activity and diet), men living at less than 500m above sea level had 5.1 times the odds (95% confidence interval [CI] 2.7 to 9.5) of being obese compared with those living at 3,000m. Women had 3.9 times the odds (95% CI 1.6 to 9.3) of being obese. Those living at more than 3,000m had an average BMI 2.4 units lower than those living at less than 500m. They found a tendency for obesity prevalence to decrease with each 200m increase in elevation, although this was not a straight line relationship.

When looking separately at the relationship between obesity and temperature, the researchers found that people tended to have lower BMI at the extremes of temperature (lower annual averages or higher annual averages), while the highest BMIs tended to be observed among those with average annual temperature around 18°C.

When looking separately at the effect of urbanisation they found that the prevalence of obesity tended to decrease with increasing urbanisation.

How did the researchers interpret the results?

The researchers conclude that obesity prevalence in the United States is inversely associated with elevation, after adjusting for urbanisation, temperature, diet, physical activity, smoking and demographic factors. Obesity prevalence is also inversely associated with urbanisation, after adjusting for these other factors – bigger cities have lower average obesity rates.

Conclusion

So would moving to a high altitude really help you lose weight? Potentially, but you’d have to leave Britain. The 3,000m height that researchers looked at is more than twice as high as Ben Nevis, Britain’s tallest mountain.

This was a large study that included a nationally representative sample of US citizens and used reliable geographic data on elevation, temperature and urbanisation. As such it was a strong study and the results can be believed.

The researchers suggest that the observed link between elevation and obesity is unknown but could be due to mechanisms such as the lower oxygen levels at high altitude, which are known to increase metabolic demands and influence hormones involved in metabolism. It could also possibly influence foetal and child growth, which could have a corresponding effect on a child’s future weight. However, the evidence of obesity levels from other mountainous countries suggests it may not be as simple as that. The relationship recorded by this research may be unique to the US.

Despite the reliable measures used in this study it does have limitations. Its cross-sectional design means that it is very difficult to conclude that altitude has a direct effect on BMI. Nor does it allow us to determine what biological process underlies the link.

Although the researchers have found that the relationship was independent of temperature, urbanisation, physical activity, diet and other lifestyle factors, as well as demographic factors (such as education and income), it is possible that the influence of all of these factors has not been completely removed or that not all factors have been considered.

It is likely that the connection between altitude and obesity is part of a complex relationship between biology, demographics, environment, lifestyle and historical factors. Due to the fast-changing demographics of the United States, the ethnic and genetic make-up of a region such as New York State (known for its large immigrant population) may be significantly different to a state such as Colorado.

One final point raised by the researchers is that, if it were proved that environmental factors associated with high altitude were responsible for weight loss then oxygen tanks could be used to replicate these conditions to aid weight loss. However, this does seem rather extreme, as would moving to a higher altitude, such as Colorado.

Despite the media headlines, the study has not examined whether, if you are overweight or obese, moving to a higher altitude country will help you to lose weight. The best advice for those wanting to shed a few pounds remains that you need to combine a healthy, balanced diet with around 150 minutes of exercise each week.